Conventionally, a device has been proposed in which by applying heat to air periodically, compressed portions of the air are formed, so that a pressure wave (compression wave, that is, an acoustic wave or an ultrasonic wave) is consequently generated (for example, see Patent Documents 1 to 4). In the device disclosed in Patent Document 1, a metal thin film serving as a heating element is formed on a substrate. By allowing a current to flow through this metal thin film, heat is generated therein and the heat is conducted to an air layer that is made in contact with the surface of the metal thin film. Moreover, in this device, a porous layer, a polymer layer or the like that has an extremely small thermal conductivity is formed between the heating element and the substrate. This porous layer or polymer layer is allowed to form a heat insulating layer and thermally insulates the heating element from the substrate. By this heat insulating layer, the flow of heat from the metal thin film to the substrate is suppressed to a low level so that the ratio of a temperature change occurring in the metal thin film relative to power of a signal that drives the metal thin film is made greater. As a result, pressure wave energy to be transmitted to the air layer made in contact with the surface of the metal thin film becomes greater, thereby making it possible to improve the generation efficiency of acoustic wave.
Moreover, Patent Document 2 has disclosed a technology in which, by changing an electric current flowing through the heating element into a periodic or non-periodic pulse state or a burst wave state that has power concentrated within a short period of time, the time-averaged power of a generated sound relative to the time-averaged applied power is improved.
Patent Document 3 has disclosed a technology in which, by forming a large number of pores are formed on a nanocrystalline silicon of the insulating layer, the degree of its porosity is set to 75% or more so that a sound pressure level is enhanced. Moreover, Patent Document 4 has disclosed a technology in which, by setting the thickness of a nanocrystalline silicon layer of the heat insulating layer to a level that is not less than a heat diffusion length specified by a frequency of an ultrasonic wave to be oscillated and is not more than a thickness obtained by adding 5 μm to the heat diffusion length, the power resistant characteristic is improved and the maximum generated sound pressure is consequently increased.
Patent Document 1: Japanese Patent Application Publication No. 11-300274
Patent Document 2: Japanese Patent Application Publication No. 2003-154312
Patent Document 3: Japanese Patent Application Publication No. 2005-73197
Patent Document 4: Japanese Patent Application Publication No. 2005-269745